22 Routes from the Sea 



for some which are partially adjusted to life on land. The pools 

 low down on a beach arc less exposed to sun, wind, rain, trickling 

 springs, and other aboceanic factors and maintain more constant 

 temperatures than those which occur at higher levels. Animals 

 which are characteristic of the former are on the whole more steno- 

 thermic than those in the latter (Klugh, 1921). Sometimes, after 

 rains or along shores where springs seep out over tide pools, a 

 curious condition is established. A pool may be separated into a 

 fresh-water stratum above and a salty, marine stratum below. The 

 pool may therefore be inhabited by certain aquatic insects, worms, 

 and other fresh-water animals living above the crabs, anemones, and 

 other animals belonging to the ocean (Gersbacher & Denison, 1930; 

 Pearse, 1914). 



An ocean beach is a difficult place to live. There are low and 

 high temperatures, strong tides, waves, desiccation, enemies which 

 come inshore when the tide rises or run down from the land when 

 the eide ebbs. Yet an abundant and varied fauna lives on ocean 

 beaches. Advantages in the way of light, food, and oxygen ap- 

 parently compensate for the disadvantages. Under the highly 

 variable conditions on the beaches some animals, such as certain 

 crabs and fishes, have undoubtedly become adapted to terrestrial 

 existence. On the California coast, MacGinitie (1935) found most 

 animals progressing toward land "through the surf, and not through 

 estuaries." 



Estuaries 



Many types of animals have entered fresh water through estu- 

 aries. These have perhaps come in part from the "mudline" which 

 borders every continental shelf just beyond the limits of wave mo- 

 tion. "From the general character of fresh-water species and from 

 the almost complete absence of free-swimming larvae, we may sup- 

 pose that the fresh-water fauna has also been derived from mud-line 



